The present invention relates to electrochemical cell stacks comprising encapsulating seals in addition to individual cell seals. The present invention also relates to improved methods of manufacturing and sealing electrochemical cell stacks through the use of individual cell seals and an encapsulating seal. The encapsulating seal is preferably be formed by injection molding or other suitable methods.
Electrochemical cells comprising polymer electrolyte membranes (PEMs) may be operated as fuel cells. In such fuel cells, a fuel and an oxidant are electrochemically converted at the cell electrodes to form a reaction product, and producing electrical power in the process. Electrochemical cells comprising PEMs may also be operated as electrolyzers, in which an external electrical current is passed between the cell electrodes, typically through water, resulting in generation of hydrogen and oxygen at the respective electrodes of the cell.
FIG. 1 illustrates a typical design of a conventional, prior art electrochemical cell comprising a proton exchange membrane, and a stack of such cells. Each cell comprises a membrane electrode assembly (MEA) 5 such as that illustrated in an exploded view in FIG. 1a. Each MEA 5 comprises an ion-conducting proton exchange membrane 2 interposed between two electrode layers 1, 3 which are typically porous and electrically conductive. Each electrode comprises an electrocatalyst at the interface with the adjacent PEM 2 for promoting the desired electrochemical reaction. The electrocatalyst generally defines the electrochemically active area of the cell. The membrane electrode assembly may be consolidated as a bonded laminated assembly.
In an individual cell 10, illustrated in an exploded view in FIG. 1b, a membrane electrode assembly is interposed between a pair of separator plates 11, 12, which are typically fluid impermeable and electrically conductive. Fluid flow spaces, such as passages or chambers, are provided between each plate and the adjacent electrode to facilitate access of reactants to the electrodes and removal of products. Such spaces may, for example, be provided by means of spacers between separator plates 11, 12 and corresponding electrodes 1, 3, or by provision of a mesh or porous fluid flow layer between separator plates 11, 12 and corresponding electrodes 1, 3. More commonly channels (not shown) are formed in the face of the separator plate facing the electrode. Separator plates comprising such channels are commonly referred to as fluid (or reactant) flow field plates.
Electrochemical cells with an ion-conductive PEM layer, hereinafter referred to as PEM cells, are advantageously arranged to form a stack 100 (see FIG. 1d) comprising a plurality of cells disposed between a pair of end plates 17, 18. A compression mechanism (not shown) is typically employed to hold the cells tightly together, to maintain good electrical contact between components, and to compress the seals. In the embodiment illustrated in FIG. 1c, each cell 10 comprises a pair of separator plates 11, 12 with MEA 5 disposed between them. Cooling spaces or layers may be provided between some or all of the adjacent pairs of separator plates in the stack assembly. An alternative configuration has a single separator plate or xe2x80x9cbipolar platexe2x80x9d interposed between pairs of membrane electrode assemblies. Such a bipolar separator plate contacts the cathode of one cell and the anode of the adjacent cell, thus resulting in only one separator plate per membrane electrode assembly in the stack (except for the end cell). In some arrangements, the stack comprises a cooling layer interposed between every few cells of the stack, rather than between each adjacent pair of cells.
The cell elements described have openings 30 formed therein which, in the stacked assembly, align to form fluid manifolds for supply and exhaust of reactants and products and, if cooling spaces are provided, for a cooling medium. Seals are typically provided between the faces of the membrane electrode assembly 5 and between each separator plate 11, 12 around the perimeter of the fluid manifold openings to prevent leakage and intermixing of fluid streams in the operating stack.
Sealing and construction of seals for electrochemical cell stacks is an important practical consideration. In some conventional cell stacks, resilient gaskets or seals are provided between the faces of the membrane electrode assembly 5 and each separator plate 11, 12 around the perimeter or at the edge to prevent leakage of fluid reactant and product streams. Such resilient gaskets are typically formed from elastomeric materials, and are typically disposed within grooves in the separator plates or membrane electrode assemblies, as described in, for example, U.S. Pat. Nos. 5,176,966 and 5,284,718. Over the course of the service life of an electrochemical cell, the elastomeric gaskets are subjected to prolonged deformation and sometimes a harsh operating environment. Over time, the resiliency of such gaskets tends to decrease due to, for example, compression set and chemical degradation, and the gaskets may become permanently deformed. This deformation impacts negatively on the sealing function and can ultimately lead to an increased incidence of leaks. Prevention of leakage and intermixing of reactants and/or coolant is an important consideration for cell stack design and manufacture. The present sealing technique overcomes problems caused by leakage of reactant and/or coolant streams from around and between gaskets, thereby improving cell stack performance.
In PEM electrochemical cells, the proton exchange membrane may project beyond the edges of the electrodes and cell separator plates around the perimeter and around manifold openings. The projecting portion of the proton exchange membrane may serve to avoid short circuits between plates, and it typically contacts and cooperates with the gaskets to form the fluid seal between the membrane electrode assembly and separator plates. Such designs tend to leave the edge of the proton exchange membrane exposed to air and/or reactant or coolant streams, however. Exposure to air or other dry gas streams can cause drying of the proton exchange membrane beginning from the edge and moving towards the center. Drying of the membrane can lead to permanent damage to the membrane, reduced cell performance and ultimately malfunction of the PEM cells. Exposure of the edge of the proton exchange membrane to some coolants and other streams can result in physical and/or chemical damage to the membrane or electrodes.
Another approach to sealing the membrane electrode assembly involves the use of an adhesive bond between each separator plate and the MEA in those regions of the cell where sealing is necessary or desirable. The adhesive bond must be substantially gas and liquid impermeable. Adhesive materials (otherwise commonly referred to as adhesives, bonding agents, glues or cements) are typically employed to form a seal and bond, for example, around the perimeter of the electrochemically active area of the MEA and around fluid manifold openings formed in the cell elements, while consolidating individual components of the PEM cell into a unitary structural unit. The MEA is preferably firmly bonded or adhered to the separator plates such that force would be required to separate the components.
In the design and manufacture of PEM cells, it is desirable to make the individual cell elements thinner. Due to the increasing demands on seals as cell elements become progressively thinner, providing for reliable sealing of fuel cell stacks will be an important part of increasing service life and decreasing maintenance costs. As cell thickness decreases, the seals between the membrane electrode assembly and separator plates have become thinner. As cells have become thinner, the cells have become more difficult to make reliably leak-proof. Further, they have become more vulnerable to electrical shorts and high temperatures within the cells. Additionally, the seals can be subjected to a harsh operating environment, which can decrease their useful service lives still further.
PCT/International Publication No. WO 00/24066 discloses a fuel cell stack in which the separator plates and membrane electrode assemblies are held together with a fluoroelastomeric adhesive/sealant. The adhesive/sealant is provided both on the outer edges of the cell components and between the membrane electrode assembly and a water transport plate. The fluoroelastomeric material is stated to replace a variety of interfacial seals and edge seals formerly required.
U.S. Pat. No. 4,774,154 discloses seal materials for fuel cells comprising a high temperature stable fluorinated elastomer and a blowing agent which activates within the range of curing temperatures of the elastomer. The elastomer is employed to seal the edges of adjacent separator plates in a fuel cell. The patent discloses that suitable elastomers are those having a range of curing temperatures which are less than to about the normal operating temperature of the fuel cell.
PEM fuel cells generate electrical power in stationary power plants, in portable power generation systems, and in the propulsion systems for motor vehicles. For these applications, a PEM fuel cell service life of at least approximately 10 years is desirable. Production cost and reliability of fuel cell seals, and simplicity and cost-effectiveness of maintenance and repair, are also important considerations.
In one embodiment, an electrochemical cell stack has a top, a bottom, and at least one side (more commonly, four sides). The stack comprises at least one membrane electrode assembly, a plurality (that is, two or more) of separator plates, at least one encapsulating seal disposed on at least one side of the cell stack, and at least one individual cell seal disposed between the membrane electrode assembly and the encapsulating seal. Each of the membrane electrode assemblies comprises an anode, a cathode and an ion exchange membrane, and each membrane electrode assembly is capable of electrochemically converting a fuel and an oxidant to produce electrical power, or (in the context of an electrolyzer) is capable of generating of hydrogen and oxygen at the electrodes. Each of the membrane electrode assemblies is disposed between two separator plates.
Each cell seal is preferably in contact with a membrane electrode assembly at or near the periphery of the membrane electrode assembly in a cell sealing area. The cell sealing area of the membrane electrode assembly divides the electrochemically active area of the membrane electrode assembly and the environment outside the membrane electrode assembly. In a preferred embodiment, the individual cell seal is disposed in a cell sealing area of the membrane electrode assembly and made of a material such that the individual cell seal will prevent leakage of reactants from the membrane electrode assembly. Alternatively, the individual cell seal need not be fluid impermeable (or gas-tight), but instead serves to prevent the encapsulating seal from contacting the membrane electrode assembly.
Each encapsulating seal is preferably in contact with the electrochemical cell stack, disposed on at least one side of the cell stack and at least between one or more pairs of separator plates that have a membrane electrode assembly between them.
In a preferred embodiment, the electrochemical cell stack is a fuel cell stack, and the ion exchange membranes are polymer electrolyte membranes.
In a preferred embodiment, one or more of the separator plates has at least one groove formed in a major surface thereof, and the encapsulating seal includes at least one rib configured to be accommodated within the groove. Alternatively or additionally, the electrochemical cell stack preferably comprises one or more coolant plates. Each coolant plate preferably comprises at least one grove, and the encapsulating seal preferably includes at least one rib configured to be accommodated within the groove.
The encapsulating seal is preferably formed from an injection moldable material. In a preferred embodiment, the injection moldable material has a curing temperature greater than the operating temperature of the electrochemical cell stack.
In an alternative embodiment, the seal is xe2x80x9cpotted,xe2x80x9d or cast in place. Potting is preferably accomplished with a thermosetting or chemical-setting seal material. One preferred form of potting is dip-molding, in which the encapsulating seal is applied by dipping the stack into the seal material or pouring a viscous thermoplastic or thermoset seal material over the outside of the stack.
In a preferred embodiment, the encapsulating seal also at least partially encases one or both end plates. One or both of the end plates preferably has at least one end plate groove formed in a major surface thereof, and the encapsulating seal preferably includes at least one end sealing portion configured to be accommodated within the end plate groove.
In a preferred embodiment, the encapsulating seal is a monolithic seal that encases each side of the electrochemical cell stack and may also additionally encase the top and bottom of the stack. The encapsulating seal preferably imparts compressive force to the electrochemical cell stack and optionally one or more of the end plate sealing portions.
In another embodiment, a compression assembly for an electrochemical cell stack consists essentially of an encapsulating seal.
An improved method is also provided for manufacturing an electrochemical cell stack. The stack has a top, a bottom, and at least one side, and, in preferred embodiments, has four or more sides. The stack comprises at least one membrane electrode assembly interposed between a plurality of separator plates. The method comprises providing an individual cell seal around each membrane electrode assembly, arranging the membrane electrode assemblies and separator plates in an operative configuration in a stack, and forming an encapsulating seal on at least one side of the stack. The encapsulating seal is disposed at least between one or more of the plurality of separator plates having one of the membrane electrode assemblies disposed between them.
In a preferred embodiment, the step of forming an encapsulating seal comprises one of injection molding and potting, which in a preferred embodiment is performed while the stack is under compression.
In a preferred embodiment, an encapsulating seal is formed while flowing a coolant through the cell stack. In this embodiment, the encapsulating seal is preferably formed at a temperature greater than the operating temperature of the electrochemical cell stack. In a preferred embodiment, the seal material has a curing temperature greater than the operating temperature of the electrochemical cell stack. The preferred method further comprises the step of preventing the encapsulating seal from contacting the membrane electrode assemblies while the encapsulating seal is being formed.